Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
The fuel fluid stream supplied to a fuel cell anode typically comprises hydrogen, which may be, for example, substantially pure gaseous hydrogen, or a dilute hydrogen stream such as a reformate stream. Other fuels such as methanol or dimethyl ether may be used instead of hydrogen. The oxidant fluid stream supplied to a fuel cell cathode typically comprises oxygen, which may be, for example, substantially pure gaseous oxygen, or a dilute oxygen stream such as air.
In solid polymer fuel cells, the water content in the reactant fluid streams supplied to and exhausted from the fuel cell may, in some cases, cause problems for conventional gas sensors. A solid polymer fuel cell employs an electrolyte that is an ion (typically proton) conductive solid polymer membrane. This membrane also separates the hydrogen supplied to the anode from the oxygen supplied to the cathode. For the solid polymer membrane to be an effective proton conductor, it must be kept sufficiently hydrated. If the membrane becomes dehydrated, in addition to reduced proton conductivity, structural failures may occur at the dehydrated portions of the membrane. For example, structural failures may result in cracks and/or holes and associated reactant leaks. Accordingly, one or both of the fuel and oxidant streams are typically humidified to ensure that these streams carry a sufficient quantity of water to prevent membrane dehydration. In addition to humidification water, the oxidant exhaust stream also typically comprises product water, which is produced by the desired electrochemical reactions that are induced at the fuel cell cathode. Accordingly, there can be a significant amount of water in the fuel cell reactant streams. For example, it is not uncommon for the water content in an oxidant exhaust stream to be about one-third by volume. The presence of such significant amounts of water in the reactant streams can hinder the operation of some conventional commercially available gas sensors, reducing the reliability and accuracy of such sensors.
Relatively low operating temperatures are another characteristic of the environment within the reactant fluid passages of solid polymer fuel cells. Generally, the temperature is less than 100° C. within the reactant fluid passages of a solid polymer fuel cell. This temperature presents a problem for conventional gas sensors which employ a solid oxide electrolyte because solid oxides are better ion conductors, and thus generally more effective, at much higher temperatures. Due to the changes in the vapor content of fluid streams in fuel cells, thermal conductivity sensors often used for ambient hydrogen detection are not generally suitable for use in fuel cell applications.
In a fuel cell, gas sensors, such as hydrogen or oxygen gas sensors may be used to monitor the respective gas concentration in the fuel and/or oxidant streams. The concentration of the reactant gases, at particular locations within the reactant streams, may be measured and used as an indicator of the fuel cell performance and operating efficiency. For example, if there is an excessive amount of gaseous hydrogen in the fuel stream exhausted from the fuel cell, this indicates poor operating efficiency, or if there is an increase in hydrogen concentration in the oxidant exhaust stream, this may be an indication of a leak in the membrane or a shortage of oxidant supplied to the cathode.
The present fuel cell assembly incorporates an improved reactant gas sensor that operates reliably and accurately when located in a fuel or oxidant fluid stream passage within a solid polymer fuel cell.